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>>>known for decades smoking is bad for you. but now the biggest study ever about the risk for smoking for women shows just how important it is to quit. the latest from
robert bazell
.

>> reporter: 28-year-old amanda
garcia
just joined a program to try to quit her smoke a day smoking pattern. a smoker since age 15. she always thought about quitting.

>>i don't think i was
ever ready
. i always said i'm not doing it until i'm ready.

>> reporter: researchers in britain followed one
million women
now in their 60s and 70s for 12 years. smoking a pack a day starting in their teens, reduced
life expectancy
by an average of 11 years. tripling the chances of a premature death. a number even higher than the risk for men.

>>if you're a smoker for your whole life, the chances that you'll die directly from your smoke rg greater than the chances of dying from everything else put together.

>> reporter: the study found that even women who smoked a few cigarettes a day doubled their chances of dying younger. but the nude from the study is not all bad. it reinforces the idea that if you quit, it reduces the risk, and the sooner the better. the research finds that if a woman who starts smoking as a teen quits before age 40, her increased chances of dying drop by 90%. even if a woman stops by age 50, her extra risk drops by 66%. for
garcia
and others 30 or younger who manage to quick, their
excess risk
disappears almost completely.
garcia
's main reason for trying to quit now, her two kids.

>>my kids mean everything to me. and i want to be there for them.

>> reporter: and she knows she has a far greater chance to be there by kicking the habit. for "today,"
robert bazell
, nbc news, new york.